1. Technical Field
The present invention relates to a printing apparatus which performs printing by ejecting ink on a printing medium from a printing head.
2. Related Art
In a serial ink jet type printer, a printing head having a plurality of nozzles is relatively moved in the main scanning direction and sub-scanning direction with respect to a printing medium, and ink is ejected from the nozzles so as to form dots on the printing medium, thereby performing printing. In such an ink jet type printer, there are cases where a position shift occurs in which a landing position of the ink ejected from the nozzle of the printing head to the printing medium is shifted from a target position. Causes of the position shift may include a problem of operation accuracy of the printing head, a problem of paper feeding accuracy, and the like. For example, if ink ejection timings of the printing head at the outward movement and inward movement are not strictly constant, a relative positional relationship between a dot group formed at the outward movement of the printing head and a dot group formed at the inward movement thereof is shifted from a target position. The shift amount is fluctuated depending on a head scanning speed or a distance variation between the printing head and the printing medium. For this reason, if cockling occurs in which a paper sheet extends and undulates at a portion where an ink ejection amount per area on the printing medium is large, a distance between the printing head and the printing medium is locally fluctuated, and thus a position shift in reciprocation scanning is also fluctuated.
If this position shift fluctuation occurs, there are cases where local density unevenness is generated in a printing image, and thereby printing quality is deteriorated. Such a density unevenness phenomenon becomes notable as a printing speed is increased. This problem is not limited to the serial ink jet type printer, and is common to a printing apparatus which forms dots through division into a plurality of pixel groups having different printing conditions and performs at least a part of the dot formation by the plurality of pixel groups at a common region, thereby outputting printing images.
JP-A-2006-49959 is an example of the related art.
JP-A-11-208031 is another example of the related art.